The invention pertains to a process for the production of silica granulate by mixing silicic acid powder with a liquid.
Silica powders can be produced by means of gas-phase reactions such as by the hydrolysis of silicon halides or organic silicon compounds. They can also be produced by means of sol-gel processes. They are obtained in large amounts as a by-product of, for example, the production of synthetic quartz glass, but it is difficult to make profitable use of such powders. Although these silica powders are characterized by a very high degree of purity, they are very difficult to handle because of their high specific surface area and their low bulk density. Nor is there any easy way to melt them down by means of known processes into transparent quartz glass components with few if any bubbles.
To facilitate the handling of finely divided silica powders and to prepare them for the production of objects of quartz glass by means of conventional production methods, it is proposed in EP-B 335 875 that the BET surface area of the silica powder be reduced by mixing the silicic acid powder with water to form a crumbly mass; to dry the wetted powder; and then to grind, sieve, and temper the dried powder.
A similar process is also known from U.S. Pat. No. 4,042,361. In the process described in this publication for the workup of synthetic silica powder, the powder is mixed with water; the wetted powder is dried; and then the powder is tempered at a temperature of 1,000.degree.-1,400.degree. C. The granulate thus produced is suitable as a feed material for the production of quartz glass crucibles by the slip-casting method.
JP 62-202,827 proposes a method for producing a silica granulate in which silica powder with a particle diameter of less than 0.1 .mu.m is mixed with a sodium-containing aqueous solution. This mixture is then dried, as a result of which agglomerates ranging in size from 50 to 500 .mu.m are obtained. These agglomerates are then sintered and vitrified.
From DD 291 445, a process for the production of a granulate to be melted for quartz glass is known, according to which the starting material is lepidoid silicic acid, which is finely ground to a particle size of less than 60 .mu.m and then dewatered at a temperature of 950.degree.-1,200.degree. C. After the addition of a 1% polyvinyl alcohol solution, the powder is finally converted by means of a sieve granulator into grains with sizes of less than 1 mm in diameter.
DE 34 06 185, a method for producing tablets of pyrogenically produced silicon dioxide particles. According to this method, a mixture of the pyrogenically produced oxide, a binder, water, and a lubricant is homogenized and pressed through a sieve. Then the granulate thus obtained is dried until it is solid enough to be press-molded. The granulate is pressed into molded articles, which are then sintered at 600.degree.-850.degree. C.
EP-A 578 553 describes a process for the production of a silica granulate by means of a sol-gel method. In the first step of this process, an aqueous suspension of silica powder is prepared, and the suspension is then gelled. After the gel has been dried, it is dispersed by microwaves. The suitable grain fraction is then sieved off.
The known processes require a large number of steps to arrive at a suitable grain size distribution, a sufficient degree of compaction, and satisfactory flow behavior of the granulate. The drying step in particular consumes a great deal of energy.